Farhan Khan (C), brother-in-law of San Bernardino shooting suspect Syed Farook, speaks at the Council on American-Islamic Relations during a news conference in Anaheim, California, December 2, 2015. Photo: Reuters/Mike Blake

A couple who were suspected of killing 14 people in a mass shooting in California and were later slain in a shootout with police were identified by authorities on Wednesday as Syed Rizwan Farook, 28, and Tashfeen Malik, 27, described as possibly married or engaged.

Armed with assault-style weapons, the two are believed to have opened fire Wednesday morning at a social services agency holiday event in San Bernardino, California. The attack, which also left 17 wounded, was the most deadly shooting in the United States in three years.

Farook was a employee of San Bernardino County who attended a holiday party at the Inland Regional Center, stormed out and later returned to fire on the celebration, said Jarrod Burguan, police chief in the city of San Bernardino, 60 miles (100 km) east of Los Angeles.

The couple are believed to be the only shooters, and while the motive remained unclear, Burguan said, "we have not ruled out terrorism."

Farook was U.S. born, while Malik's nationality was still unknown.

The shooting rampage on the campus of an agency that serves the developmentally disabled marked the deadliest U.S. gun violence since the massacre at Sandy Hook Elementary School in Newtown, Connecticut, in December 2012, in which 27 people, including the gunman, were killed.

Wednesday's carnage differed from most other recent U.S. killing sprees in key ways, including the involvement of two people rather than a lone perpetrator.

It also comes less than three weeks after the deadly attacks in Paris prompted tighter security at many public venues across the United States.

At a news conference called by the Los Angeles area chapter of the Muslim advocacy group Council on American-Islamic Relations (CAIR), the brother-in-law of Farook, Farhan Khan, said he was bewildered by the news.

"Why would he do that? Why would he do something like this? I have absolutely no idea, I am in shock myself," Khan said at a news conference in Anaheim, California, south of Los Angeles.

Hussam Ayloush, executive director of CAIR in the Los Angeles area, said Farook and his wife had been missing since Wednesday morning.

So far in 2015, there have been more than 350 shootings in which four or more people were wounded, according to the crowd-sourced website shootingtracker.com, which keeps a running tally of U.S. gun violence.

The shooting in California comes less than a week after a gunman killed three people and wounded nine in a shooting rampage at a Planned Parenthood clinic in Colorado Springs, Colorado. In October, a gunman killed nine people at a college in Oregon, and in June, a white gunman killed nine black churchgoers in South Carolina.

Gun control advocates, including Democratic President Barack Obama, say easy access to firearms is a major factor in the shooting epidemic, while the National Rifle Association and other pro-gun advocates say the Second Amendment of the U.S. Constitution guarantees Americans the right to bear arms.